This invention relates to novel molding methods for the manufacture of thermoplastic lenses and is particularly adaptable for the manufacture of steeply curved ophthalmic lenses and ophthalmic lenses with thin centers. The invention includes the resultant lenses.
Lens Molding
Lenses are used for a variety of purposes, for example in optical devices such as microscopes and eye glasses. Over the past few years, the use of thermoplastic material to prepare ophthalmic lenses for such uses as in vision corrective and in prescriptive (Rx) spectacle lenses as opposed to traditional glass lenses has increased dramatically because thermoplastic lenses offer several advantages over glass. For example, plastic is lighter than glass and hence spectacles with plastic lens are more comfortable to wear especially since the nominal lens thickness is typically 2.0-2.2 mm. Other factors for increased demand for thermoplastic lenses are that these lenses can be made scratch and abrasion resistant, they come in a wide range of fashionable colors, and because the production techniques have improved so that they can now be manufactured at higher rates and in a more automated fashion.
Of the thermoplastic lenses, the use of polycarbonate thermoplastic is becoming very attractive as compared to, for example, lenses made from individual casting and thermoset-peroxide curing allylic resins. Factors favoring polycarbonate thermoplastic lenses include lower density and higher refractive index than cast-thermoset plastic. Hence, thinner lenses in the range of 1.5-2.0 mm thickness can be made. In addition, polycarbonate lenses of the same nominal thickness as thermoset-peroxide cured allylic resins will be of lighter weight, due to lower density, and therefore will impart greater wearer comfort. Furthermore, polycarbonate thermoplastic lenses have far greater impact and breakage resistance than any other optical grade polymeric material.
Heretofore, thermoplastic, injection-molded lenses have been manufactured by injection molding with or without any compression. Injection molding without any compression typically involves the use of a mold cavity having fixed surfaces throughout the molding cycle. Such molding processes employ very long molding cycles, high mold-surface temperatures, higher than average plastication and melt temperatures for that given resin, and slow controlled fill rates followed by very high packing pressures which are held until gate freeze-off is complete.
Fixed cavity processes of the type described above, employ larger than normal gating and runner systems to permit maximum packing pressure and delivered material before gate freeze-off occurs, at which time no further transfer of molten polymer occurs between the runner system or plasticating unit and the cavity. Gate freeze-off in a fixed cavity injection machine presents a problem, given that powered lenses have differing front and back radii of curvature, prescription lenses must therefore have differing cross-sectional thicknesses which in turn leads to non-uniform shrinkage during part formation in the mold cavity and cooling-down which can cause poor optics and/or distortion. In addition, the thickest sections of the lens are subject to slight sink marks or depressions which in turn cause a break in the otherwise uniform radius of curvature of the lens surface. This break results in a localized aberration or deviation in the light bending character of the lens at that area of sink.
Thus, although great care is taken to see that the injected polymer mass conforms perfectly to the fixed lens mold cavity surface, contour, and dimensions, once gate freeze-off prevents additional packing pressure and material transfer, differential shrinkage begins to occur within the polymer melt and the polymer skin begins to pull away from the mold surfaces accordingly. This pre-release detrimentally affects optical quality since the molded lens contour and surface no longer can be forced by intimate contact to exactly replicate the precision optical mold surfaces and cure contours. Also, a fixed cavity molding process is limited in how thin the lens center can be. Below about 2 mm, the molten plastic preferentially flows around the thick edge leaving a void and/or knit line which extends into the central zone of the to-be-formed lens.
To address these problems with fixed cavity molding processes, compression molding techniques have been used. The injection/compression molding techniques are divided into two types (i) the clamp-end injection/compression and (ii) the auxiliary component injection/compression. In the clamp-end injection/compression method, the molten polymer is injected into a mold space formed by moving the mold platens and mold halves to a predetermined position. After or during injection, the molten polymer mass is allowed to cool for a predetermined time interval and the injection molding machine commences a closing motion of the movable platen. This clamping-up motion compensates for shrinkage occurring during freezing of the molten polymer. Under this clamp-induced compressive force, the mold cavity""s contents continue cooling and solidifying, eventually reaching a temperature sufficiently below the glass-transition temperature, or freezing point, of the injected polymer that the molded article may be safely ejected without risking optical distortion. However, in view of the high clamp pressure, thin centered lenses may be subjected to crushing of the frozen center portion while the remaining areas of the mold retain molten polymer.
This method however, has severe limitations. First, it is critical to carefully control the injection pressure and fill rate, along with the timing interval. For example, the injected melt must be allowed to form a surface skin and partially solidify to prevent molten polymer from spilling outside the desired runner-mold-cavity configurations, necessitating costly and laborious trimming operations on the molded part. Second, if the melt solidifies to too great an extent, compression at ultimate clamping pressures can cause hobbing or deformation of the mating plats at the parting line, thus damaging the mold set. Third, if compression is delayed too long, too much polymer solidification will have occurred when the compressive force through final clamp-up is initiated, resulting in forcible reorientation of the polymer and cold working of the plastic, which, in turn, produces birefringence and undesirable molded-in stress levels, with resulting localized nonuniform light-bending characteristics.
In the auxiliary component injection/compression method the compressive pressure is applied to the opposing optical surfaces via auxiliary springs, cylinders or the like which are either internal to the mold itself or as peripheral apparatus thereto. Early thermoplastic lens molding of this type employed simple spring-loaded, movable optical dies within the mold set. Johnson, et al., xe2x80x9cCompressor Unitxe2x80x9d, U.S. Pat. No. 2,443,826, issued Jun. 22, 1948. Such apparatus created a variable volume lens mold cavity thereby, but relied upon high internal polymer melt pressure to spread the movable dies against the resisting spring pressure. In order to apply sufficiently great compressive forces upon the solidifying mold contents, these spring forces were great. However, the greater the spring force, the greater the injection pressure that must be used to compress the springs during variable cavity fill. The greater the injection pressure required, the greater the degree of molded-in stresses and optically unsatisfactory birefringence. The greater the optical power for the molded lens, the greater the dissimilarity between the front and back curves and thus the greater the cross-sectional thickness variation. Therefore, this process is limited to production of weakly powered lenses with minimal diameter and minimal thickness variations.
Another auxiliary component process is represented by the patents to Weber: xe2x80x9cApparatus for Injection Molding Lensesxe2x80x9d, U.S. Pat. No. 4,008,031, issued Feb. 15, 1977; and xe2x80x9cMethod for Injection Molding Lensesxe2x80x9d, U.S. Pat. No. 4,091,057, issued May 23, 1978. Weber teaches a variable-volume cavity formed by injection-melt, pressure induced rearward deflection of at least one movable male or female die which after a certain interval is followed by forward displacement resulting in compression under the driving force of an auxiliary hydraulic cylinder mounted in one-to-one relationship with this movable die. Flow ports are provided through which excess polymer melt is forcibly extruded from the lens cavity under the compressive forces. Weber too relies upon a preset amount of time to elapse between completion of injection fill and commencing compressive pressure. Therefore, this process too suffers from defects caused by premature compression or excessively delayed compression discussed above. Additionally, this process can produce lenses of inconsistent thickness.
Another auxiliary component process is described by Laliberte. Laliberte, xe2x80x9cMethod for Molding Ophthalmic Lensesxe2x80x9d, U.S. Pat. No. 4,364,878, issued Dec. 21, 1982. This process includes a movable die coupled to an auxiliary hydraulic cylinder. After the mold is closed under clamp pressure, the mating die parts are spread apart by injection of a polymer. A fixed amount of polymer, adequate to fill the fully compressed mold-cavity system is then injected. This process permits greater control of nominal lens thickness and therefore eliminates material scrap waste and trimming operations. However, Laliberte discloses lens thickness control but only with regard to nominal 3.0 mm center thickness which is significantly greater than the desired consumer lens thickness.
Another major short-coming of the injection/compression molding processes described above is that they are unsuitable for manufacturing Rx lenses, especially minus thermoplastic lenses having a center thickness of about 1 mm or less and having edge thicknesses greater than the center thickness. This is because the injected thermoplastic melt in the thinner center portion of the minus lens freezes prior to the freezing of the melt in the thicker edge portions. As a result, the compressive pressures generated by the mold halves (optical inserts) at this point of solidification is focused only on the frozen center portion which crushes or otherwise distorts this part of the lens. Such crushing or distorting of the frozen center is particularly problematic at center thicknesses of about 1 mm or less and having molten edge thicknesses substantially larger since the entire compressive force is concentrated on a small diameter, thin column of frozen material at the center. Also, this force exceeds the compressive strength of the solidified material. However, as is apparent, thin centered minus lenses having a thickness of about 1 mm or less are particularly desirable as having still further reduction in weight as compared to conventional minus lenses having a center thickness greater than about 1 mm (e.g., 1.5 mm).
In view of the inability of injection/compression molding processes to prepare thin centered minus lenses, such lenses have been manufactured by abrading and polishing thicker lenses. Such manufacturing techniques employ abrading and polishing elements such as optical curve generating, fining and polishing machines which inevitably leave abrasion/polishing residues on the lens surface and/or leave negative fining marks below the nominal surface.
Steeply Curved Lenses
Applicants have been involved in the development and manufacture of various types of steeply curved lenses, including unpowered lenses, sun lenses and prescription plus or minus lenses. Such lenses have a higher curvature than conventional Ostwalt lenses in all or a significant portion of their usable surfaces. Steeply curved lenses have the potential of providing many advantages including eye protection and increased field of view. Such lenses may be used in cosmetically desirable eyewear.
Various types of steeply curved, wide field spherical lenses are disclosed in the above-mentioned patent application Ser. No. 09/223,006. Other types of highly curved lenses are disclosed in International Application number PCT/AU99/00430 to Morris et al., (International Publication No. WO 99/63392) the contents of which is hereby incorporated by reference in its entirety. Among other things, these applications disclose novel ophthalmic lenses, all or a portion of which have steep base curvatures. For example, Ser. No. 09/223,006 discloses a powered ophthalmic lens having at least one spherical surface with a radius of curvature less than about 35 mm. As another example, International Application number PCT/AU99/00430 discloses various lens shapes including xe2x80x9cbowlsxe2x80x9d and xe2x80x9covaliformsxe2x80x9d, at least portions of which are steeply curved. Such lenses are characterized by large sagittal or hollow depths.
Applicants have investigated methods for fabricating steeply curved lenses by employing molding processes. Though some usable lenses were made, difficulties were encountered in fabrication of such lenses using conventional molding processes due to the steep curves, large sagittal depths and thin regions encountered in these lens designs.
Accordingly, it is an object of the present invention to provide methods for effectively and economically molding steeply curved and/or thin-centered lenses.
It is another object of the present invention to provide steeply curved and/or thin-centered lenses lacking surface defects or internal defects produced by conventional molding processes.
These and other objects and features of the present invention will be apparent from this written description and associated drawings.
This invention is directed to novel molding methods for manufacturing steeply curved and/or thin centered thermoplastic lenses. The methods are particularly well adapted for the manufacture of lenses having a center portion of thickness less than about 2 mm. The methods of this invention can produce thermoplastic lenses without knit lines, witness marks or excessive stress in the thinnest portions.
In preferred embodiments, this invention relates to molding methods for the preparation of steeply curved lens elements with thin centers. The methods involve overfilling of the mold cavity followed by compression of the lens mold sections prior to freezing of the thermoplastic melt. Subsequently, the mold halves are maintained in place while pressure is increased on the thermoplastic melt to compensate for the shrinkage which occurs during solidification of the melt in the mold. Subsequent cooling of the mold results in formation of a lens element with novel properties.
The methods of this invention are particularly advantageous in that a molding process is employed wherein crushing of the thin lens by the molds during manufacture is avoided. The methods may also avoid the formation of knit lines and witness marks in such lenses.
Accordingly, in one of its method aspects, this invention is directed to a method for manufacturing a thermoplastic lens element which comprises:
(a) providing a mold comprising movable sections wherein said sections, when closed, define a mold cavity in the form of a thermoplastic lens element having an optical surface at least a portion of which has a maximum principle curvature in a local region characterized by a radius of curvature of less than about 35 mm;
(b) introducing into the mold cavity a molten thermoplastic material in a quantity at least sufficient to form the lens element;
(c) moving at least one of said mold sections to a predetermined hard stop point and stopping said at least one mold half at said hard stop point prior to freezing of the thermoplastic material at the thinnest point of the to-be-formed lens element;
(d) maintaining said mold halves in a stationary position at said hard stop point while controlling pressure in the mold cavity to maintain a constant volume within the mold; and
(e) permitting the thermoplastic material to freeze thereby forming the steeply curved thermoplastic ophthalmic lens element.
In one embodiment, the mold pressure in the mold cavity is increased by injection of further melted thermoplastic resin into the mold. In another embodiment, this increase in internal cavity pressure is achieved by an injector, or by use of one or more screws, secondary pistons, pins, or other mechanisms.
In a second of its method aspects, this invention provides a method for manufacturing a thermoplastic lens element which comprises:
(a) providing a mold with movable sections which when moved to predetermined positions define a mold cavity in the shape of a steeply curved lens element to be molded;
(b) introducing into the mold a volume of molten thermoplastic material in excess of the volume of the steeply curved lens element to be molded;
(c) prior to the cooling of the thermoplastic material at its thinnest point below its glass transition temperature, moving the mold sections relative to each other to a position to define a cavity of a volume approximately equal to that of the steeply curved lens element to be molded; and
(d) maintaining the mold sections in said position while applying hydraulic pressure to the thermoplastic material in the mold until the thermoplastic material in the mold freezes into the steeply curved lens element being molded.
The present invention also includes an injection molded, steeply curved lens element having an optical surface at least a portion of which has a radius of curvature less than about 35 mm, the lens elements being formed without knit lines or witness marks in the central optical portion of the lens. In preferred embodiments, the lens element is a lens blank adapted for making an edged lens with large sagittal or xe2x80x9chollowxe2x80x9d depth. The lens blank may be semispheric having at least one spherical surface with a radius of curvature less than about 35 mm.
In the case of a negative powered lens, the lens element may have a thin central section with a minimum thickness less than 2 mm. The lens element may have at least one optical surface which lies within a spherical shell defined by two concentric spheres having radii whose lengths differ by no more than 2 mm, the smaller of the radii being no more than 50 mm in length and wherein at least two points O and Q on the edged lens subtend an angle OPQ greater than 80xc2x0 with respect to the center P of the shell.
Using the designs of application Ser. No. 09/223,006, the present methods may, for example, be used to mold lens elements which in preferred embodiments have a base curve of 16.0 Dxc2x1approximately 0.5 D and which exhibit relatively low RMS power error over at least approximately forty degrees of eye rotation from optical center. Such lens elements may be made in the form of edged lenses having a through power from the temporal to nasal edge of the lens varying by no more than 0.5 D from the prescription power.
The foregoing has been provided as a convenient summary of certain aspects of the present invention. However, the invention intended to be protected is defined by the claims and equivalents thereof.